The invention relates to a method of preparing a poly-crystalline silicon film, and, more particularly, a method of preparing a poly-crystalline silicon film by a dehydrogenation treatment of amorphous silicon deposited by a plasma chemical vapor deposition process.
Amorphous silicon thin film transistors (a-Si TFTs) have been used in conventional liquid crystal display devices as pixel switching elements. The a-Si TFT includes an active layer made of the a-Si film that can be uniformly deposited at a relatively low temperature on a glass substrate by a plasma-enhanced chemical vapor deposition (PECVD) process. The glass substrate, which has a lower heat-resistance, is suitable for making a display region larger in area.
Driver-circuit integrated-type liquid crystal display devices have been recently developed. In this type of liquid crystal display device, driver circuits are disposed surrounding the display region (pixel region) on the same substrate as the pixel switching elements. TFTs are also used for the driver circuits in addition to the switching elements. TFTs for the driver circuits especially require a high-speed response characteristic. Since poly-crystalline silicon (p-Si) films are remarkably higher immobility than a-Si films, p-Si TFTs with active layers made of p-Si films are used for the driver-circuit integrated-type liquid crystal display devices.
Poly-crystalline silicon films are usually formed in the following way. First, an a-Si film is deposited on a glass substrate by a PECVD process. Next, a dehydrogenation treatment is carried out on the a-Si film. In the dehydrogenation treatment, the glass substrate upon which the a-Si film is deposited is received in a heating chamber in a low pressure atmosphere and is annealed at a lower temperature (e.g., 400xc2x0 C. through 500xc2x0 C.) than a heat-resistance temperature of the glass substrate. The dehydrogenation treatment discharges hydrogen from the a-Si film. Finally, an excimer laser beam is used to irradiate the a-Si film, melting and resolidifying the a-Si film and changing it into a p-Si film.
The a-Si film deposited on the substrate by the PECVD method contains a great quantity of hydrogen. When laser beams are used to irradiate the a-Si film without the dehydrogenation treatment, hydrogen is discharged so abruptly from it that a phenomenon called xe2x80x9cablationxe2x80x9d takes place, forming detects in the p-Si film.
Conventionally, a heating chamber or furnace is separately provided for the dehydrogenation treatment in addition to a PECVD system. The dehydrogenation treatment in the heating chamber takes a rather long time, for example, up to several hours including heating and cooling time of the glass substrate. Further, the glass substrate with the a-Si film is washed before dehydrogenation treatment because the substrate is exposed in the atmosphere between the PECVD process and the heating chamber.
Because of the background stated above, the dehydrogenation treatment for the a-Si film in the production process of the p-Si film has been one of the critical factors hindering the productivity of p-Si films.
An object of the present invention is to provide efficient dehydrogenation treatment for an a-Si film deposited by a PECVD process to produce a p-Si film.
In a method of preparing a p-Si film of the invention, a glass substrate is placed in a process chamber of a PECVD system. Plasma discharge is performed in an atmosphere of reactive and carrier gases in the chamber to deposit an a-Si film on the substrate. When the a-Si film has been deposited on the substrate, the pressure of the chamber is reduced with respect to the atmospheric pressure but the substrate is left in the chamber to perform dehydrogenation treatment for the a-Si film. After the dehydrogenation treatment is complete,the substrate is taken out from the chamber. Laser beams irradiate the a-Si film on the substrate, transforming it into a p-Si film.
According to the method of preparing the p-Si film of the present invention, after the a-Si film is deposited on the substrate, the dehydrogenation treatment is continuously carried out while the substrate is kept in the reaction chamber. Since the substrate is heated sufficiently when the a-Si film is deposited on the substrate, it is unnecessary to heat the substrate again before executing the dehydrogenation treatment. Cooling treatment of the substrate is conducted after completion of the dehydrogenation treatment. Thus, time required for reheating and recooling the substrate in the conventional method can be reduced in the method of the present invention. In addition, since the substrate is not exposed to the atmosphere between the deposition and dehydrogenation treatments, it is not necessary to wash the substrate before the dehydrogenation treatment. As a result, the total processing time in the method of the present invention can be shortened significantly.
Preferably, after the a-Si film is deposited on the substrate in the process chamber of the PECVD system, the substrate remains in the process chamber to carry out the dehydrogenation treatment while the carrier gases are supplied to the chamber. Further, after the a-Si film deposition process, the substrate is kept in the chamber at a pressure that is set higher than the pressure of the deposition process. Generally, the dehydrogenation treatment is carried out more efficiently as its temperature rises. If the flow of gases remains unchanged with the substrate in the chamber and the pressure of the chamber is set higher than the pressure of the deposition process, the temperature of the substrate rises rapidly and the hydrogen is discharged from the a-Si film.
Preferably, after the deposition of the a-Si film, the substrate is left in the chamber and the output power of a heater for the substrate is kept the same as during the deposition process. If, at the time the process is changed from the PECVD to the dehydrogenation, the pressure of the chamber is set higher than during the PECVD process, the flow of gases is unchanged in the chamber, and the output power of the heater is maintained, the adjustment time for the temperature in the chamber to start the next PECVD process can be shortened.
Furthermore, preferably, the substrate temperature at the deposition process is set at 400xc2x0 C. or more. The period of time t (seconds) during which the substrate is left in the chamber is set to satisfy the following equation (1):
t greater than d2/(Axc3x97exp B)xe2x80x83xe2x80x83(1)
where
A=6.0xc3x971014,
B=xe2x88x922.56xc3x9710xe2x88x9219/(kxc3x97(273+xcex8) ),
k=1.38xc3x9710xe2x88x9223,
d (angstroms) is the thickness of the a-Si film and
xcex8 is the substrate temperature when the substrate is left in the chamber.
Equation (1) can be derived from the following theory. The rate of the dehydrogenation can be determined by the diffusion of hydrogen in the a-Si film. When the root mean square diffusion length of hydrogen xe2x80x9cxcexxe2x80x9d is greater than the thickness xe2x80x9cdxe2x80x9d of the a-Si film, dehydrogenation can be brought about. Where the diffusion coefficient is D (cm2/sec) in the a-Si film and the leaving period of time is t (sec), the root mean square diffusion length xcex (cm) during the leaving period of time is given by the following equation:
xcex=(2xc3x97Dxc3x97t)+xc2xd
The above stated condition (i.e., the relationship between the thickness d (angstroms) of the a-Si film and the root mean square diffusion lengths (cm)) can be expressed by the following equation:
d less than (2xc3x97Dxc3x97t)+xc2xdxc3x97108
From that equation, the leaving period of time required for efficient dehydrogenation treatment is expressed by equation (2):
t greater than d2 /(1016xc3x972xc3x97D)xe2x80x83xe2x80x83(2)
where
D=Doxc3x97exp (xe2x88x92Ed/kT),
Do=3xc3x9710xe2x88x922 (cm2/sec),
Ed=1.6xc3x971.60xc3x9710xe2x88x9219 (J),
Ed: hydrogen diffusion energy (1.6 eV),
T: temperature (K) of the a-Si film, and
k: Boltzmann""s constant (1.38xc3x9710xe2x88x9223 J/K).
Inserting these values into equation (2) results in equation (1). The method of the present invention is carried out particularly effectively when a single substrate processing type PECVD system with a small heat capacity is employed.
According to the present invention, a p-Si thin film manufacturing process time can be greatly saved as set forth hereinafter.
First, a method of preparing a p-Si film of the present invention includes the steps performed in a PECVD system in which a PECVD process is carried out to form an a-Si film on a glass substrate. The reaction chamber of the process is heated to a temperature of 400xc2x0 C. or higher. Dehydrogenation remarkably takes place in the chamber in that temperature range. The substrate is then taken out from the reaction chamber and is received and maintained in a heating chamber for further dehydrogenation treatment. A period of processing time necessary for post-heating the substrate for such further dehydrogenation treatment can be shortened more effectively than the time necessary for conventional processes in which dehydrogenation treatments are conducted in separate heating chambers provided independently from a PECVD system.
Second, it is substantially sufficient to perform a cooling treatment only once after the dehydrogenation. As a result, reheating and recooling time periods can be decreased relative to conventional processes.
Third, substantially no substrate washing treatment is needed because the substrate is not exposed to air after the completion of a-Si film forming.
Finally, it brings about an additional time saving effect for dehydrogenation because a hydrogen content in an a-Si film is reduced from the beginning of a film forming process due to a glass substrate kept at 400xc2x0 C. or higher.
The above-stated ends, other objects, and technical features of the present invention will become apparent from the following description when taken with the accompanying drawings. It will be understood, however, that the drawings are for purposes of illustration and are not to be construed as defining the scope of the invention, reference being had for the latter purpose to the claims appended hereto.